1. Field of Invention
The present invention relates to a process using radiofrequency microwave energy to enhance gaseous releases from solutions, such as performing wet calcination and its analogues.
2. Background
Wet calcination is a common procedure to process ores containing bicarbonates where the desired products are carbonates. It has been applied to the processing of trona ores whose chemical composition is largely Na.sub.2 CO.sub.3.NaHCO.sub.3.2H.sub.2 O, and where the desirable product is soda ash, Na.sub.2 CO.sub.3, since this is a valuable industrial chemical.
The common processing of trona ore proceeds with a material size reduction of the ore which is then treated in solution either by other chemicals or heat or both to break down the bicarbonate. Further selective crystallization is employed to purify the final soda ash.
Trona is normally mined in a conventional manner producing an ore containing various impurities which after conventional material size treatment is then dissolved into a water solution for purification by crystallization. However future efforts are likely to employ solution mining underground and eliminate the conventional mining costs and problems. Thus a water solution of trona is directly available to begin the processing steps.
An excellent treatise of the procedures and problems associated with modern trona mining and processing is by H. W. Haynes, Jr., Solution Mining of Trona, IN SITU, 214(4), 357-394 (1997), which is hereby incorporated by reference, and is hereafter referred to as Haynes.
The preferred treatment of the bicarbonate in trona is to decompose it into soda ash, and since sodium carbonate is produced, the procedure is often called calcination and the equipment a calciner. The basic reaction is: EQU 2 NaHCO.sub.3 .fwdarw.Na.sub.2 CO.sub.3 +CO.sub.2 +H.sub.2 O(1)
Thermodynamically this reaction takes a minimum of 70.degree. C. but to obtain a reasonable rate of reaction temperatures from 150 to 200.degree. C. are employed in the calciner. However for trona ores often higher temperatures than about 200.degree. C. are not feasible to further enhance the desired reaction as the various impurities, such as silicon dioxide, react with the soda ash producing undesirable byproducts, such as sodium silicates. Thus trona calciner temperature below 200.degree. C. are utilized resulting in long processing times. However by leaching of the trona before calcination the silicon dioxide is largely eliminated allowing increased flexibility. A number of solution problems in obtaining and purifying such soda ash are well documented by Haynes.
The analogues of wet calcination consist of solution chemical decomposition reactions where one of the important reaction products is a gas. For wet calcination the product was carbon dioxide which is released upon the decomposition of bicarbonates. A similar process occurs when sulfur dioxide is released upon the decomposition of bisulfite.
The importance of the decomposition of bisulfites occurs because of the hazardous nature of sulfur dioxide so it is not freely released into the atmosphere. Solutions of sodium sulfite and sodium bisulfite are produced when solutions of soda ash, sodium carbonate, or its equivalent such as trona mine water, are utilized as a scrubbing agent to remove sulfur dioxide from flue gases. The sodium sulfite is stable but the sodium bisulfite slowly releases sulfur dioxide into the atmosphere, and this is undesirable. Therefore for the stabilization of these solutions it is necessary to convert the bisulfite to sulfite under controlled conditions where the released sulfur dioxide is contained.
The reaction is: EQU 2NaHSO.sub.3 .fwdarw.Na.sub.2 SO.sub.3 +SO.sub.2 +H.sub.2 O(2)
This is the analogue of Eq.(1) with the atom sulfur replacing the atom carbon.
The primary purpose of the subject invention is to enhance the rates of Eq.(1) and Eq.(2) and other analogues by the use of microwaves.
Quantum radiofrequency (RF) physics is based upon the phenomenon of resonant interaction with matter of electromagnetic radiation in the microwave and RF regions since every atom or molecule can absorb, and thus radiate, electromagnetic waves of various wavelengths. The rotational and vibrational frequencies of the electrons represent the most important frequency range. The electromagnetic frequency spectrum is usually divided into ultrasonic, microwave, and optical regions. The microwave region is from 300 megahertz (MHz) to 300 gigahertz (GHz) and encompasses frequencies used for much communication equipment. For instance, refer to Cook, Microwave Principles and Systems, Prentice-Hall, 1986.
Often the term microwaves or microwave energy is applied to a broad range of radiofrequency energies particularly with respect to the common heating frequencies, 915 MHz and 2450 MHz. The former is often employed in industrial heating applications while the latter is the frequency of the common household microwave oven and therefore represents a good frequency to excite water molecules. In this writing the term "microwaves" is generally employed to represent "radiofrequency energies selected from the range of about 500 to 5000 MHz", since in a practical sense this total range is employable for the subject invention.
The absorption of microwaves by the energy bands, particularly the vibrational energy levels, of atoms or molecules results in the thermal activation of the nonplasma material and the excitation of valence electrons. The nonplasma nature of these interactions is important for a separate and distinct form of heating employs plasma formed by arc conditions of a high temperature, often more than 1650.degree. C., and at much reduced pressures or vacuum conditions. For instance, refer to Kirk-Othmer, Encyclopedia of Chemical Technology, 3rd Edition, Supplementary Volume, pages 599-608, Plasma Technology. In microwave technology, as applied in the subject invention, neither condition is present and therefore no plasmas are formed.
Microwaves lower the effective activation energy required for desirable chemical reactions since they can act locally on a microscopic scale by exciting electrons of a group of specific atoms in contrast to normal global heating which raises the bulk temperature. Further this microscopic interaction is favored by polar molecules whose electrons become easily locally excited leading to high chemical activity; however, nonpolar molecules adjacent to such polar molecules are also affected but at a reduced extent. An example is the heating of polar water molecules in a common household microwave oven where the container is of nonpolar material, that is, microwave-passing, and stays relatively cool.
In this sense microwaves are often referred to as a form of catalysis when applied to chemical reaction rates. For instance, refer to Kirk-Othmer, Encyclopedia of Chemical Technology, 3rd Edition, Volume 15, pages 494-517, Microwave Technology.
Related United States microwave patents include:
______________________________________ U.S. Pat. No. Inventor Year ______________________________________ 4,076,606 Suzuki et al. 1978 4,671,951 Masse 1987 5,451,302 Cha 1995 ______________________________________
Referring to the above list, Suzuki et al. disclose a process for decomposing NO.sub.x in a gaseous medium using microwave energy. However the subject invention while performing a decomposition reaction using microwaves employs a water medium.
Masse discloses purifying liquid sulfuric acid by the evaporation of water employing microwave energy. The subject invention does not involve such a purification process but does employ a microwave cavity reactor.
Cha discloses a process of microwave catalysis of chemical reactions using waveguide liquid films. The concentration of phosphoric acid by removal of bound water and the release of carbon dioxide from pregnant solutions of monoethanolamine are shown. However the subject invention does not utilize such a waveguide liquid film which requires a microwave-passing substrate.